backend: Cleanup cursor tracker on shutdown

It keeps references to cursors, and cursors keep references to DRM
buffers. In order to be able to clean up on exit, explicitly destroy the
cursor tracker on shutdown.

We can't rely on GObject reference counting, as gjs might hold onto a
reference until it's garbage collected.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1822>

src/backends/meta-backend.c

@@ -199,6 +199,7 @@ meta_backend_dispose (GObject *object)
   MetaBackend *backend = META_BACKEND (object);
   MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
 
+  g_clear_pointer (&priv->cursor_tracker, meta_cursor_tracker_destroy);
   g_clear_object (&priv->current_device);
   g_clear_object (&priv->monitor_manager);
   g_clear_object (&priv->orientation_manager);